Optical devices for optical channel filtering, such as wavelength add/drop filters, bandpass filters, directional couplers, etc, are important elements of optical telecommunication systems.
In Wavelength Division Multiplexing (WDM), a plurality of N mutually independent optical signals are transmitted along a line. Each of these independent signals has a respective transmission wavelengths λ1, λ2 . . . , λN, which are different from each other and each of which defines a transmission “channel”. All these wavelengths, which have a spacing between them, should be included in a useful amplification bandwidth of optical amplifiers used in the line.
However, for such signal to be useful, it must be possible to wavelength selectively drop the optical signal coming in through an optical fiber (or through another optical conduit) to a fiber/conduit conveying to the respective user/destination and to wavelength selectively add signals to the fiber. Therefore, efficient adding and dropping of channels, without modifying the other optical channels, is even more crucial in DWDM (Dense Wavelength Division Multiplexing) where a very high number of channels at slightly different wavelengths is transmitted in a single fiber simultaneously.
For this purpose, it is desirable to realize filters which may add or drop one or more signals in a certain bandwidth. An example of such a filter is disclosed in U.S. Pat. No. 4,737,007 to Alferness. In this patent a narrow-band, wavelength selective optical coupler is described, which includes two optical fibers, each comprising a core region and a cladding region, and a Bragg diffraction grating formed in the coupling region of the two fibers.
In the following, only couplers including a grating, which is a periodic structure formed by spatially varying refractive index distribution throughout a defined volume or the boundary of a guiding region, are considered.
In particular, considering two waveguides 1 and 2 having effective refractive indices n1 and n2, respectively, being in close proximity to each other so as to form a directional coupler, and a periodic perturbation (grating) inserted in the coupling region such that its wave-vector obeys to one of the following relationships
                                                    K            +                                    =                                            2              ⁢              π                                      Λ              +                                =                                                    β                1                            +                              β                2                                      =                                                            2                  ⁢                  π                                                  λ                  0                                            ⁢                              (                                                      n                    1                                    +                                      n                    2                                                  )                                                                        (        I        )                                                                K            -                                    =                                            2              ⁢              π                                      Λ              -                                =                                                    β                1                            -                              β                2                                      =                                                            2                  ⁢                  π                                                  λ                  0                                            ⁢                              (                                                      n                    1                                    -                                      n                    2                                                  )                                                                        (        II        )            where Λ is the grating periodicity, β1 and β2 are the propagation constant in waveguides 1 and 2 at λ0, there is coupling between the two waveguides only at the specific wavelength λ0 named above. If Eq. (I) is satisfied, then the wavelength λ0 is coupled in the backward direction from one waveguide to the other and the directional coupler is said to be a contra-directional coupler, whereas if Eq. (II) is satisfied, the wavelength λ0 is coupled in the forward direction and the coupler is said to be a co-directional coupler.
From the above equations (I) and (II), it is clear that, given a transmission signal including a plurality of channels having different wavelengths (λ1, . . . , λn) propagating in the first waveguide, the filtered wavelength to the second waveguide isλ0=Λ(n1±n2).  (III)
A particularly desirable additional characteristic of optical couplers is wavelength tunability, so that the dropped wavelength may be changed, in order to increase the flexibility of networks. The goal of a tuneable coupler is therefore to select one channel (or several channels) in a given incoming input optical signal and transmitting all other channels through the filter, said channel being changeable.
Several different tuneable optical filters have been developed. A proposed solution has been to realize the core region of one of the two waveguides in a tuneable material, i.e. a material whose refractive index may be changed, therefore changing n1 or n2. This implies, a change in the filtered wavelength λ0 (see eq. (III)).
Silica on its own may be thermo-optically tuned. However its thermo-optic coefficient dn/dT is of the order of 10−5/° C. and a change of temperature of 100° C. will typically shift the filter wavelength by less than 1 nm. This may restrict the applications where the desirable tuning range is of several nm.
In “Thermooptic Planar Polymer Bragg Grating OADM's with Broad Tuning range”, published in IEEE Photonics Technology Letters, vol. 11 (1999), p. 448, a tuneable add/drop multiplexer having a grating printed in a single-mode polymeric waveguide with a thin-film heater is described. Thermal tuning can be achieved by a large thermo-optic coefficient dn/dT of −3·10−4/° C.
Applicants have noticed that employment of polymeric materials with high dn/dT in the waveguide core region makes polymer stability a crucial issue and may affect long-term reliability of the coupler. Furthermore, for an OADM (Optical Add/Drop Multiplexer) consisting of a waveguide and two optical circulators, the high cost of the circulators makes the device too expensive.
In “Polarisation insensitive and tuneable optical add and drop multiplexer utising vertically stacked buried semiconductor waveguides”, published in Electronics Letters vol. 35, No. 20, p. 1733-1734, a tuneable, vertical and contra-directional coupler filter, in which specially designed buried semiconductor waveguides are used, is disclosed. In particular, the two waveguides are made of InGaAsP, are buried in a InP cladding and a grating is formed between the waveguides in the filter region. The tunability is achieved by changing the temperature.
Applicants have noted that, in order to obtain a tuning of several nm, for example of 10-11 nm, the temperature of III-V semiconductors such as InGaAsP should be varied of at least 500° C., which implies a huge thermal variation of the overall device, which can be detrimental for the device performances. They have further observed that fabrication technology based on III-V semiconductors is rather complex and expensive.
In “Hybrid silica-polymer structure for integrated optical waveguides with new potentialities”, published in Material Science and Engineering vol. B57, p. 155-160, a coupler comprising two coplanar waveguides having silica cores and a polymer cladding is described. More specifically, the directional coupler of this paper comprises two ridge cores made of silica which are covered by the same polymer cladding. A Bragg grating is UV-induced in both waveguides in the central part of each sample. The negative thermal optical coefficient for the polymer refractive index enables the authors to auto-stabilise the Bragg grating in temperature.
Applicants have noted that in the described coupler a variation in temperature induces the same variation of effective refractive index in the two waveguides with the aim of stabilizing the coupled wavelength and to render it independent of the temperature. This device would be unsuitable for making a tuneable directional coupler.
U.S. Pat. No. 6,097,865 to Alferness et al. discloses a wavelength filter having a low index waveguide, a high index waveguide, having substantially different geometries which results in substantially different effective indices, and a grating for coupling therebetween. Both waveguides are made of InGaAsP. In this patent, the so obtained high effective index differential Δn=n2−n1 of the filter waveguides is desired in order to achieve a high bandwidth and reduce cross-talk.